NASA Studies Potential Fix for Discovery's Heat Shield
High-resolution images like this taken of Discovery by ISS astronauts allowed engineers to detect the protruding gap fillers. Click <a href="http://spaceflight.nasa.gov/gallery/images/station/crew-11/hires/iss011e11084.jpg">here</a> to access a high-resolution view of this image, where one gap filler can be seen protruding from a region just behind the orbiter's nose landing gear doors.
Credit: NASA/JSC.

This story was updated at 8:40 p.m. EDT.

HOUSTON - NASA engineers are drawing up plans to remove two strips of filler material jutting out from the heat-resistant tiles the space shuttle Discovery belly should mission managers decide the action is needed to safeguard the orbiter's heat shield, shuttle officials said Sunday.

The potential repair, a first if implemented, could be folded into the last of three planned spacewalks for Discovery's crew on Aug. 3, or even warrant a fourth extravehicular activity (EVA), though it is still undecided whether any action is required, they added.

"The jury is out, at this point, on whether any we'll do anything," said Wayne Hale, NASA's deputy shuttle program manager, during an afternoon briefing at here at Johnson Space Center (JSC). "Just the fact that we know about this situation is something new and completely different."

Hale said the imaging provided by astronauts aboard the International Space Station (ISS) - where Discovery is currently docked - and the shuttle's orbital inspection boom have given flight controllers they're first glimpse of the heat shield protrusions, gap-fillers made of a ceramic fiber cloth. In past shuttle flights, they have only been found after landing, shuttle officials said.

While space shuttles have landed safely numerous times with such protrusions, aerodynamics experts are working to determine how the protruding gap-fillers would affect Discovery's reentry through Earth's atmosphere during its planned Aug. 8 landing.

"We have a team of folks working aggressively on options to make that gap filler safe if we decide it's an issue," said Paul Hill, lead flight director for Discovery's STS-114 mission, adding that a separate team is studying the reentry heating effects involved with leaving the protrusions in place. "We expect to have final results on aeroheating and a decision on whether we need to do anything about the gap fillers on Monday."

The aeroheating results should be among of the last pieces of data NASA needs to give Discovery a clean bill of health. NASA engineers are currently working to complete their analysis of Discovery's wing leading edges to ensure the heat-resistant reinforced carbon carbon panels (RCC) are safe for reentry. Initially projected for completion today, image analysts have asked for an additional day to finish studying data collected by Discovery astronauts during a follow-up inspection, Hale said.

The shuttle's ceramic tiles and thermal blankets were cleared for reentry Saturday, NASA officials said.

"We think Discovery is safe to bring home," NASA's top administrator Michael Griffin said on NBC's "Meet the Press" Sunday. "We have approximately one-sixth the number of scars on this orbiter, by actual count, as compared to the average of the last 113 flights."

During in-flight inspections of Discovery's heat shield, astronauts aboard the International Space Station (ISS) photographed the two gap-fillers sticking out; one about 1.1 inches (2.7 centimeters) out from just behind the nose landing gear and another farther back jutting between six-tenths (1.5 centimeters) to nine-tenths (2.3 centimeters) of an inch into space. The protrusions were not caused by impacts or other damage and have been seen in past shuttle flights, but can cause increased drag during reentry and hotter temperatures just aft of the filler materials, NASA officials said.

A typical shuttle reentry generates temperatures of about 2,300 degrees Fahrenheit along the orbiter's tile-covered underside.

During NASA's STS-73 shuttle flight aboard Columbia in 1995, an errant gap-filler was found jutting from its heat shield after landing, said Steve Poulos, NASA's orbiter projects manager. Measuring about 1.4 inches (3.5 centimeters) unrolled, the gap-filler actually stuck out about 0.6 inches (1.5 centimeters) during the descent, with reentry temperatures for the tile section in that flight were estimated at about 2,700 to 2,800 degrees Fahrenheit, he added.

Hill said that gap filler protrusions from the shuttle's forward tile sections are understood out to about a quarter-inch (0.6 centimeters). Since the largest protrusion seen on Discovery is about an inch long, it warrants the added study, he said.

A spacewalk activity

If a fix is needed, it appears relatively straightforward.

One of Discovery's two spacewalkers, mission specialists Soichi Noguchi and Stephen Robinsons, could snip away the excess gap filler or pull it out entirely while standing at the end of a robotic arm, but whether that arm is attached to the shuttle or ISS is still under discussion.

Both astronauts have rehearsed some gap-filler removal techniques as part of the thermal protection repair method training they received for their mission's first spacewalk, Poulos said. Removing both gap-fillers entirely, if needed, would not compromise the shuttle's heat shield, he added.

The station's arm could be repositioned from its current location outside the Destiny laboratory to its Mobile Base Platform, which would allow more clearance between the arm and Discovery during any fix operation, Hill added.

"If it's relatively simple...why wouldn't you not go out and take care of it," Hale said of the potential spacewalk repair.

Discovery's arm could reach the two gap fillers - which are located behind the shuttles nose landing gear doors - but is currently carrying the 50-foot (15-meter) orbital boom sensor system (OBSS) used for heat shield inspections. Shuttle arm operators would have to hand off the boom to station arm controllers for stowage in Discovery's payload bay, where it could block access to other equipment.

"So the options get complicated," Hill said.

One option not on the table calls for placing a spacewalker at the end of Discovery's orbital boom, an operation planned to be tested in a future shuttle flight, Poulos said.

If mission managers decide any fix is needed, it will likely be tacked onto Discovery's third spacewalk - which currently has about 30 minutes spare time thanks to get ahead activities performed but Noguchi and Robinson in a July 30 spacewalk - instead of a fourth EVA since flight controllers hope to dedicate an extra mission day announced Saturday to additional cargo transfer, Hill added.

Meanwhile, Discovery astronauts continue to haul cargo into the ISS from their orbiter's mid-deck and a cargo pod they delivered to the station.

"I can't be happier with the progress of this mission and I couldn't be happier with the performance of this crew and the flight control team," Hill said.

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